[docs] decision: storage exposes a typed cookie API and storage_state save/restore

[AutomatedTester]

💥 What does this PR do?

Proposes a design decision record: storage exposes a typed cookie API and storage_state save/restore.

BiDi cookies use an awkward wire shape (value is a BytesValue descriptor, partitioning via a descriptor), so the common "set a session cookie" case carries needless ceremony. And the most-requested workflow — log in once, capture state, restore it to skip login in later sessions (Playwright's storage_state) — has no Selenium equivalent. This record proposes a typed Cookie value for set_cookie/get_cookies plus a save_state/load_state pair, while being explicit that cookies come from the BiDi protocol whereas localStorage is captured via script.evaluate and IndexedDB is out of scope.

🔧 Implementation Notes

• Follows the docs/decisions/ process from [docs] add design decision record process and template #17665 — depends on [docs] add design decision record process and template #17665 landing (adds only docs/decisions/0003-*.md).
• Cookies are fully BiDi; web storage is not in the spec, so storage_state captures localStorage by evaluating script per origin — the record documents this split deliberately rather than implying uniform protocol support.
• No wire-protocol change.

🤖 AI assistance

• AI assisted (complete below)
  • Tool(s): Claude Code
  • What was generated: Initial draft of the decision record, derived from a CDDL-validated comparison of the Python BiDi surface against Playwright; revised through discussion and review
  • I reviewed all AI output and can explain the change

💡 Additional Considerations

One of a set of BiDi ergonomics decision records proposed together. The cookie convenience and storage_state are kept in one record as a single cohesive storage-ergonomics decision; reviewers who prefer them split can request that. Cross-binding convergence is tracked in the binding-status table.

🔄 Types of changes

• Documentation (design decision record)